1. Field of the Invention
This invention relates to the field of energy management. More specifically, the invention comprises a system for matching the usage of energy by a first device to the production of energy being supplied by a second, separate device.
2. Description of the Related Art
The present invention is applicable to a diverse array of energy management applications. One particularly suitable application is the management of so-called “green” energy sources such as photovoltaic solar arrays. The invention is best understood by discussing specific applications, so a solar power example will be used throughout this disclosure. With that concept in mind, the reader may benefit from a brief discussion of existing solar collection technology.
Photovoltaic solar arrays convert sunlight into relatively low-voltage direct current. This electrical energy may be used directly, converted to another form (such as alternating current) or stored for future use. FIG. 1 shows an example of direct use. In this example, energy produced by a solar array is applied to a swimming pool circulation pump. A pool pump must be run during times when the sun is not shining. Thus, a conventional line powered pump 14 is retained in the system. Auxiliary pump 28 is added by plumbing it in parallel. Valves 32, 34, 36, and 38 are used to select between the two available pumps.
Auxiliary pump 28 is typically powered by a DC motor which is fed directly from solar array 24 by DC power line 30. When the sun is shining, the valves are positioned so that auxiliary pump 28 draws water in through pump inlet line 40 and discharges it through pump outlet line 42. When the sun in not shining, the valves are set to use pump 14.
Of course, a system such as depicted in FIG. 1 can never use solar power to run the auxiliary pump when the sun is not shining. In order to provide such a capacity, an energy storage means is needed.
Chemical batteries are typically used to store the energy. In this approach, the photovoltaic array is connected to a charge controller which regulates the flow of electricity into and out of the batteries. It is a simple matter for the charge controller to charge the batteries when the output of the photovoltaic array is greater than the battery voltage. However, some sophisticated charge controllers use pulse width modulation techniques to step up the available voltage from the solar array so that charging can continue even during the off-peak daylight hours—albeit at a lower current. These same controllers are able to provide a near-optimum charging voltage to extend the battery life. In this fashion, energy may be stored during peak sunlight hours and used to power loads when desired.
Many facilities using solar energy are tied to the conventional power grid. When the sun is shining, the available solar electricity is converted to alternating current in an inverter and used to power one or more selected AC circuits in the facility. At night, energy stored in chemical batteries can drive the inverter and this energy can also be used to power selected circuits. The circuits being powered by the solar energy must typically be isolated from the AC power in the rest of the facility so that amplitude and phase matching of the supplied solar energy is not required.
Unfortunately, the batteries in existing solar energy systems have proven to be the shortest-lived components. Photovoltaic arrays can have a useful life of twenty years or more. Being solid state devices, they require little maintenance. Chemical batteries—on the other hand—typically provide two to five years of service life. This may be greatly shortened if the batteries are drained too deeply. Some specialized battery designs offer longer life, but these tend to be very expensive. Thus, a solar energy system having no batteries would be a significant advantage. Such a system represents one application of the present invention.